Sildenafil is a selective inhibitor of phosphodiesterase whose chemical name is 1-[4-ethoxy-3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo[4,3-d]pyrimidin-5-yl)phenylsulphonyl]-4-methylpiperazine. This compound and its preparation method as well as its use in treating cardiovascular diseases was disclosed in CN1124926A; CN1057464A disclosed the use of this compound in preparing medicine for treating erection dysfunction of male animals. CN1168376A disclosed a new method for preparing sildenafil. CN1246478A disclosed another method for preparing sildenafil. Although sidenafil is very effective on treating male erectile dysfunction, the compound has strong t toxicity and side effects.